There are many problems associated with lean fuel combustion or low BTU gas combustion in gas turbine engines. With the successful flight of the X-43A, hypersonic flight has achieved several technological goals and entered a new era. With further developments, it will reach Technology Readiness Level. However, FAA and EPA emission regulations in addition to the cost of fossil-based aviation fuels have pushed the aviation community into research and development for highly efficient aircraft engines that work on alternative and/or renewable fuels. In particular, the land based gas turbine community has been conducting research for integrating coal gasification with a combined cycle turbine (“IGCC”). In any case, the combustion of the product from gasification of carbon containing matter—synthesis gas (“syngas”)—requires major modifications to current gas turbine engines. Because syngas has a low heating value (“LHV”) compared to natural gas, significantly more fuel must be injected in an IGCC turbine than a natural gas turbine. Therefore, the mass-flow—and thus the output power—of the gas turbine is much higher for an IGCC application. For the same reason, the gas turbine's output power is flat-rated to very high temperatures.
Supersonic Combustion and Flame Holding
Problem 1: Gas Turbine to Ramjet or Scramjet Operations
High Bypass Fan Gas Turbines are the primary engines for transportation aircraft. Typical speeds are 893 km/h (482 kt) at altitude on aircraft such as the Boeing 777-300. Military aircraft use augmentors (“afterburners”) to achieve and sustain supersonic flight. Only the new F-22 raptor can sustain supersonic flight without the use of an augmentor. Air breathing ramjets or scramjets are required to achieve hypersonic flight using air. However, only one successful Scramjet has been flown since the beginning of aviation. The major problem with ramjets and scramjets can be traced back to the early problem of flame holding or preventing engine flame out. In addition, no matter which configuration is chosen for Hypersonic Flight a problem remains—transition from subsonic to supersonic and finally hypersonic flight will require several different engines.
Problem 2: Lean Combustion or Low BTU Fuel Combustion
Fuel-lean combustion can increase efficiency while lowering emissions. However, current combustors cannot hold a flame during lean combustion conditions. Likewise, low BTU fuel such as syngas is difficult to combust in current gas turbine engines.
Ansaldo Energia (Genoa, Italy) has engineered a new gas turbine V94.2K2 that targets the low-Btu (3.5 MJ/Kg-7 MJ/Kg LHV) market. The K2 gas turbine builds on the design philosophy of Ansaldo Energia's V94.2K that can handle fuels with 8 MJ/Kg-13 MJ/Kg LHV. The K2 is intended for Chinese and Eastern European markets where the company sees a demand for power generation from industrial gases, such as Blast Furnace Gas (BFG) and Coke Oven Gas (COG).
The Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL) for common hydrocarbon based fuels, such as diesel (0.6% to 7.5%), gasoline (1.4% to 7.6%), and natural gas (Methane—5.0% to 15%) is fairly limited in range. On the other hand, syngas a product of natural gas (methane) steam reforming or gasification of hydrocarbons, coal, biomass, etc. is composed of hydrogen and carbon monoxide. The LEL and UEL for hydrogen (4% to 75%) and carbon monoxide (12.5% to 74.0%) are much broader than the parent fuel such as methane. Thus, this allows syngas (hydrogen+carbon monoxide) to be burned in a lean mode. The problem with syngas is that it is not widely available. It must be generated onsite by steam reforming natural gas or gasification of carbonaceous matter such as hydrocarbons or biomass. Typical gasifiers are very large and extremely expensive. However, a small and inexpensive plasma gasifier, such as the ArcWhirl®, U.S. Pat. No. 7,422,695 issued on Sep. 9, 2008 to the present inventor coupled to an IC engine could achieve both lean burn and supersonic combustion by gasifying the fuel first then combusting it in a cyclone combustor that is driven by a turbocharger.
Problem 3: Match Lit in Hurricane
Supersonic combustion has been compared to keeping a match lit in a hurricane or tornado. All gas turbines slow the flow of compressed air to below supersonic velocities in order to maintain a flame. This is due to the inherit design of the flame-holding capabilities of the combustor for a given turbine.
Move Match to Eye of Hurricane
It is well known that speeds within a cyclone can easily attain supersonic velocities. For example, turbochargers and centrifugal compressors easily attain speeds of over 100,000 RPM. Likewise, the air circulating within a turbocharger would far exceed supersonic speeds. Thus, the present invention achieves supersonic combustion by utilizing the centripetal forces within a rotating air column, such as a cyclone or hurricane for energy transfer, while utilizing the void, commonly referred to as the eye or vortex, in order to keep the match lit in order to maintain ignition. Simply put, the match is moved from the whirling column of air known as the shear wall to the eye or center of the hurricane.
Problem 4: Flame is Stretched Due to Whirl Flow & Melts Turbine
Placing the igniter within the center of the combustor is common for many types of gas turbine engines. Allison's C-18 to C-20 series of gas turbine engines utilize a front mounted axial flow compressor that sweeps the compressed air to the combustor via externally mounted air conduits. If the combustor were redesigned such that the air tubes entered tangentially to the combustor housing, thus creating centrifugal flow within the redesigned cyclone or vortex combustor, then the igniter and fuel nozzle would be placed within the central void space or eye of a whirling mass of air. However, this would create a detrimental effect on the compressor turbine if operated at supersonic combustion utilizing an intense igniter such as a plasma torch. The intense heat within the centrally stretched out plasma flame would melt the center of the compressor turbine.
Problem 5: Flame Out
It is well known that lean combustion can achieve high efficiency while producing low emissions. However, attempting to achieve lean combustion within current internal combustion (“IC”) engine designs may lead to low reaction rates, flame extinction (“Flame Out”), instabilities, and mild heat release. Likewise, many IC engines are very sensitivity to fuel/air mixing.
With the current push for sequestering carbon or utilizing renewable fuels, a need exists for a relatively inexpensive turbine engine design that can operate in a lean fuel combustion mode in addition to a supersonic combustion mode. If such a turbine could be easily coupled to a motor generator, high bypass fan or propeller, this would allow for rapid transition to renewable fuels for electrical generation, aviation, marine propulsion and thermal oxidation. The ability to transition from subsonic to supersonic then to hypersonic flight with the same engine would solve many problems with reaching space at an affordable payload rate. The ability to use the same air breathing supersonic combustion turbine as a steam plasma thruster in space solves the issues of carrying a large oxidizer payload.